1. Field of the Invention
The present invention relates to a solid-state imaging device and method, and more particularly to a solid-state imaging device having an overflow drain (hereinafter referred to as OFD) structure for draining off an excess electrical charge generated in a photodiode.
2. Related Art
A first example of prior art is the horizontal overflow drain structure shown in FIG. 8 to FIG. 10, of which FIG. 8 is a plan view of the structure, FIG. 9 is a cross-sectional view of the structure along the cutting line Z-Zxe2x80x2 of FIG. 8 and the potential distribution in the operating condition, and FIG. 10 is a cross-sectional view along the cutting line U-Uxe2x80x2 of FIG. 8 and the potential distribution in the operating condition.
In the first prior art example of FIG. 9, the difference with respect to the present invention is the existence of a shutter gate 34 over the gate oxide film 48, the shutter gate 34 is a gate of a surface channel transistor in the P well 42. The other elements of the first layer polysilicon gate 31, the readout gate 33, the P+ channel stopper 43, the charge transfer N well 44, and the interlayer insulation film 51, are the same as in the structure of the present invention. Control of OFD operation is done by applying a voltage to the shutter gate 34, so as to drain off electrons 49 in the photodiode 45 through a surface channel.
A second example of prior art is the vertical-type overflow drain structure shown in FIG. 11 to FIG. 13, of which FIG. 11 is a plan view thereof, FIG. 12(a) is a cross-sectional view along the cutting line V-Vxe2x80x2 in FIG. 11, FIG. 12(b) is a potential distribution diagram in the operating condition, and FIG. 13 is a potential distribution diagram in the operating condition along the cutting line W-Wxe2x80x2 of FIG. 12, in which the upper line is the potential when electrons 79 are accumulated in the photodiode N well 76, and the lower line is the potential when the electrons 79 accumulated in the photodiode N well 76 are read out to the charge transfer N well 74 by applying a voltage to the readout gate 63 so as to raise the surface potential of the P well 72.
In a vertical-type overflow drain structure, photodiode N well 76 on the opposite side from the charge transfer N well 74 is in contact with the P+ channel stopper 73, and the electrons 79 accumulated in the photodiode N well 76 are controlled by the voltage applied to the N-type substrate 71. In this case, in order that electrons from parts other than the photodiode N well 76 are not pulled out to the N-type substrate 71, a second P well 80 is provided. The other elements of a first layer polysilicon gate 61, a readout gate 63, a charge transfer N well 74, and an interlayer insulation film 81, are the same as the structure of the present invention.
In a solid-state imaging device of the prior art as described above, however, there are the following problems.
First, in the first prior art example, in order to increase the red sensitivity of the part made up of the photodiode N well 46 and photodiode cap layer 37 of the photodiode part, it is necessary to form an N-type region as far as a deep position (1 to 2 xcexcm) in the substrate, so that there was a tendency for variations to occur in the potential of the photodiode part (potential B in FIG. 9) during operation. For this reason, by considering the manufacturing margin in the impurity concentration in the photodiode N well 46, it was necessary to make the shutter gate voltage of the OFD part high.
In the second example of prior art, because it is necessary to create a second P well 72 at a deep position (3 to 4 xcexcm) within the substrate, there was a tendency for variations to occur in that position. For this reason, there was a tendency for variations to occur in the substrate applied potential for pulling electrons 79 away from the photodiode N well 76. Additionally, it was necessary to apply a high voltage (15 V or higher) to the substrate.
Accordingly, it is an object of the present invention to provide a solid-state imaging device with a vertical-type overflow drain structure, which reduces the variations in the amount of accumulated charge in the photodiode part, and that enables the drain off of electrons accumulated in the photodiode with a low control voltage and having good repeatability.
To achieve the above-noted object, the present invention has the following basic technical constitution.
Specifically, a first aspect of the present invention is a solid-state imaging device comprising; an opto-electrical conversion well of a first conductivity type formed on a substrate, a separation layer of a second conductivity type to separate the opto-electrical conversion well of the first conductivity type so as to form a plurality of photodiodes, a cap layer of the second conductivity type formed on a surface of the opto-electrical conversion well, a charge drain control layer of the second conductivity type formed within the opto-electrical conversion well of the first conductivity type, a photodiode well and a charge drain well of the photodiode formed by providing the charge drain control layer, formed within the opto-electrical conversion well of the first conductivity type.
In a second aspect of the present invention, the cap layer, the separation layer, and the charge drain control layer have depths that increase in this sequence.
In a third aspect of the present invention, the cap layer, the separation layer, and the charge drain control layer have impurity concentrations that increase in this sequence.
In a fourth aspect of the present invention, a width W1 of the separation layer is greater than a width W2 of the charge drain control layer.
In a fifth aspect of the present invention, a width of the charge drain control layer is formed to be greater than 1 xcexcm so as to obtain an overflow drain operation mode device.
In a sixth aspect of the present invention, a width of the charge drain control layer is formed to be at least 1 xcexcm so as to obtain an shutter operation mode device.
A method of the present invention is a method of solid-state imaging device comprising an opto-electrical conversion well of a first conductivity type formed on a substrate, a separation layer of a second conductivity type to separate the opto-electrical conversion well of the first conductivity type so as to form a plurality of photodiodes, a cap layer of the second conductivity type formed on a surface of the opto-electrical conversion well, a charge drain control layer of the second conductivity type formed within the opto-electrical conversion well of the first conductivity type, a photodiode well and a charge drain well of the photodiode formed by providing the charge drain control layer, formed within the opto-electrical conversion well of the first conductivity type, wherein the method comprising the steps of; a first step of forming the separation layer by ion implantation, a second step of forming the cap layer by ion implantation, and a third step of forming the charge drain control layer by ion implantation over the separation layer.